This invention relates generally to heat exchangers and more specifically to a heat recuperator in which the temperature of heat transfer tubes contained therein generally does not fall below the dew point of the heating gas. A particularly useful application of such a recuperator is to preheat air in Compressed Air Energy Storage (CAES) systems.
CAES power plants have become effective contributors to a utility's generation mix as a source of peaking or intermediate energy and spinning reserve. CAES plants store off-peak energy from relatively inexpensive energy sources such as coal and nuclear baseload plants by compressing air into storage caverns such as underground reservoirs.
Following off-peak storage, the air is later withdrawn from storage, heated, combined with fuel and expanded through turbine generators to provide needed peaking/ intermediate power. Since inexpensive off-peak energy is used to compress the air, the need for premium fuels, such as natural gas and imported oil, is reduced by as much as two thirds compared with conventional gas turbines.
For a more complete discussion of CAES systems, see Nakhamkin, N. et al. "Compressed Air Energy Storage: Plant Integration, Turbomachinery Development", ASME International Gas Turbine Symposium and Exhibition, Beijing, Peoples' Republic of China, 1985 and Nakhamkin, M. et al. "Compressed Air Energy Storage (CAES): Overview, Performance and Cost Data for 25MW to 220MW Plants", Joint Power Generation Conference, Toronto, Canada, 1984, both incorporated herein by reference.
To further increase the efficiency of CAES systems, such systems are provided with heat recuperators which can further reduce premium fuel consumption by as much as twenty-five percent. A recuperator utilizes low pressure turbine exhaust heat as the heating gas for preheating compressed air from the storage cavern prior to entering a high pressure combustor. The high pressure combustor combines the preheated compressed air with fuel and outputs the resulting products of combustion to a high pressure turbine which drives a generator to produce power. Exhaust gas from the high pressure turbine is combined with additional fuel in a low pressure combustor and the products of combustion are directed to a low pressure turbine. The low pressure and high pressure turbines are located on the same shaft and are connected to each other via appropriate gearing, and to the generator via a clutch. The exhaust gas comprising products of combustion of compressed air and fuel from this low pressure turbine is utilized as the heating gas in the recuperator.
The high temperature medium used in such recuperator, namely, the products of combustion (i.e., exhaust gas) output by the low pressure turbine is at approximately atmospheric pressure and generally has a significantly lower heat transfer coefficient than that of the low temperature medium which is high density compressed air illustratively at a pressure between 20 bar and 100 bar, and typically 60 bar. Due to this dissimilarity, the tubes' temperature is much closer to the temperature of the high density low temperature medium than to that of the high temperature medium. As a result, in conventional counterflow recuperators, the tubes' temperature is typically below the acid dew point of the low pressure turbine's exhaust gas, especially in the area where heat transfer takes place between cooled exhaust gas exiting the recuperator and cold compressed air. Since the exhaust gas contains SO.sub.x, this enters in reaction with the condensed water on the exterior surface of the tubes, forming sulfuric acid, which in turn causes severe corrosion and ultimately failure of the tubes in the recuperator.
This problem of corrosion is especially of concern in CAES systems due to compressed air temperatures as low as approximately 120.degree. F., significantly less than the acid dew point of the exhaust gas, which illustratively is 250.degree. F. Conventional gas turbine systems generally do not encounter such corrosion since the coldest relevant air temperature, illustratively 700.degree. F., is significantly above the acid dew point.
Various corrosion resistant materials have been suggested to combat this severe problem of corrosion. However, test results indicate that even the most expensive corrosion resistant materials would not last longer than 2000 operating hours. (Lukas, H. "Corrosion of Materials in a Simulated CAES Exhaust Gas Environment", ASME Gas Turbine Conference and Exhibit, Houston, Texas 1985).
An additional proposed solution is disclosed in U.S. Pat. No. 4,523,432 to Frutschi. However, such a solution requires the use of additional equipment including a separate hot water storage device to initially preheat the compressed air prior to entry into the recuperator.
Although known devices to combat corrosion do exist, deficiencies associated with long-term corrosion prevention remain. Present use of corrosion resistant materials and additional preheaters external to the recuperator clearly does not adequately remedy these deficiencies. Corrosion prevention without the need for additional costly preheating devices is most critical to proper operation of any heat recovery system susceptible to corrosion.